1. Field of the Invention
The present invention relates generally to a method and device to perform wavelength modulation and more specifically to a method and system for controlling current injection into a Distributed Bragg Reflector (DBR) semiconductor laser to perform wavelength modulation.
2. Background
Lasers have been employed in display technologies for years. In displays such as computer displays, televisions, or the like, colors are generated by the superposition of three primary colors: red, blue and green. As such, within laser-based displays, lasers are employed to provide the primary colors. Each laser can be raster-scanned across the screen or can be stationary and employed to illuminate an image (e.g., a motion picture film or spatial light modulator containing an image). The ability of a laser to provide a beam having excellent brightness characteristics leads to efficient and well-performing lasers within laser-based projectors, when compared to the brightness characteristics of incandescent bulbs used in conventional motion picture theaters.
DBR semiconductor lasers can be used for laser-based displays, among other applications, as they can provide efficient wavelength conversion. For example, a 1060 nm DBR semiconductor laser tuned to a spectral center of a second-harmonics-generation (SHG) device such as a non-linear crystal may be used to convert the wavelength output by the DBR semiconductor laser to a 530 nm beam. This provides a low-cost, compact and efficient non-linear source of green light. Generally, for technologies involving video displays, the optical power such as that used to generate the intensity of green light, for example, needs to be modulated at a fundamental frequency of approximately 50 MHz and with an extinction ratio of approximately 40 dB. The extinction ratio is the ratio of high optical power level to low optical power level. To achieve this combination of high modulation speed and larger extinction ratio remains a daunting task.
One way to obtain a DBR laser and second harmonic generator (SHG) based light source having a fast modulation and a large extinction-ratio is to rapidly modulate the output wavelength of a DBR semiconductor laser. As a result, the DBR semiconductor laser beam rapidly scans cross the narrow spectral width of a non-linear SHG device to produce the necessary intensity modulation. For example, if maximum green power is needed, the DBR wavelength is tuned to the spectral center of the non-linear crystal while, if zero green power is needed 10 ns later, the DBR wavelength is tuned outside the spectral width of the non-linear crystal to provide a dark image.
FIG. 1A schematically illustrates a conventional DBR semiconductor laser 100 and a second harmonic generation (SHG) device 150. The DBR semiconductor laser 100 includes a DBR potion 110, a phase portion 120 and a gain portion 130. The gain portion 130, when injected with a continuous wave (CW) current, generates continuous optical power for the laser. The current injected into the DBR potion 110 makes large changes to wavelengths output from the laser and the current into the phase portion 120 makes small changes to the wavelength of the beam output of the laser. The SHG device 150 receives the beam produced by the semiconductor laser 100, whose output intensity of the converted wavelength (green, for example) depends upon alignment of the DBR laser wavelength and the SHG device's spectral center. The beam output from the SHG device 150 is then directed to an output such as display screen.
The simplest way to rapidly tune the DBR semiconductor laser's output wavelength is by injecting modulated current into the DBR portion and phase portion of the DBR semiconductor laser 100 while keeping the gain-portion current continuous and constant. As illustrated in the chart provided in FIG. 1B, a video signal can require green light with an intensity of up to 100% within each bit period of the signal. The bit period width is the inverse of the system frequency, for example, the resident time of each pixel of a raster scan on a display screen. For the example shown in FIG. 1B, an intensity of 100% is the brightest possible signal while 0% is dark. Thus, as illustrated in FIG. 1B, the video intensity required for the first bit period is 100%, the intensity reduces to 0% for the second bit period and is increased to 40% for the third bit period.
With conventional systems, the current injected into the DBR portion 110 of the laser is pulse width modulated based on the required intensity in each bit period. That is, the duration within one bit period in which the current is “on” is proportional to the intensity of the video signal in that bit period (shown in the first waveform from the top of FIG. 1B). Ideally, the wavelength of the output of a DBR semiconductor laser is shifted based on the carrier induced effect and output to the SHG device 150 (shown in the second waveform from the top of FIG. 1B). The SHG device 150, based upon the received beam, outputs a converted beam having an ideal intensity signal for display, as illustrated in FIG. 1B. However, the simple scheme described above ignores the possible adverse thermal effect that the injection of current into the laser causes.
Generally, current injection generates two effects within DBR semiconductor lasers. First, a carrier effect is generated that provides more carriers in the portion increasing carrier density and reducing the refractive index within the DBR portion or the phase portion. As a result, a shorter wavelength beam is generated. Current injection also causes a heating effect which causes the temperature of the semiconductor laser device to rise. Currents higher than zero cause a temperature rise in the DBR portion and the phase portion of the laser, thereby increasing the refractive indices, which tend to generate a longer wavelength beam. The collective wavelength shift is produced by the combined effect of the carrier effect and thermal effect. For large current values that are needed to achieve large wavelength shift, the temperature rise is severe enough to reduce and sometimes completely reverse the carrier-induced wavelength shift.
FIG. 2 illustrates the effect that the injection of current, and the resulting increase in temperature, can have on the operation of the laser. Specifically, FIG. 2 shows the wavelength shift provided by the laser as a function of the DBR-portion current-pulse width, measured at the end of each current pulse width. The wavelength shift induced by the carrier effect is approximately −0.6 nm. This is shown in the lower left portion of the graph where the pulse width is approximately 150 ns or less. However, when the pulse width of the injection current increases beyond 150 ns, the heating effect discussed above begins to reduce the effects of the carrier effect. In fact, if the current pulse becomes long enough, the carrier effect becomes entirely negated by the heating effect.
Another drawback of current induced thermal effect is that it provides a slow wavelength modulation process. The thermal effect, which causes the temperature of the laser to increase, has μs- to ms- characteristic time compared to the carrier effect that has ns- carrier lifetime. The degree of thermal effect also depends upon the current amplitude and the heat sinking conditions associated with the laser. The slow response of the thermal effect is also illustrated in FIG. 2, as the wavelength shift does not change for pulse widths of less than 150 ns. Slow thermal effect results in an undesirable patterning effect because the average heating depends upon the width of pulses and therefore on the pattern of the video signal. In other words, the DBR semiconductor laser wavelength at a particular bit of the video signal depends on the history of the previous bits of data.
The adverse effects resulting from a temperature rise in the laser are also shown in the chart of FIG. 1B. Specifically, when injection current is applied to the DBR portion 110 of the DBR semiconductor laser 100, and the current is constantly on, the DBR temperature rises as shown by the DBR temperature waveform in FIG. 1B. As a result, the actual DBR wavelength waveform provided from the laser to the SHG device 150 will be distorted, and the resulting output from the SHG device 150 will also be distorted, and the required intensity of the original video signal is not achieved at the output of the SHG device 150.